This invention relates to a bicycle derailleur, and more particularly to an improved derailleur structure applicable to a rear derailleur which is used for selectively shifting a drive chain from one sprocket to another sprocket of a multi-speed freewheel comprising usually three to eight different sized sprockets. The structure is also applicable to a front derailleur which is used for shifting the chain from one gear to another gear of a multi-speed chain wheel comprising usually two to four different sized gears.
The conventional rear derailleurs, that are now actually available in the commercial markets, generally have a parallelogram linkage mechanism for guiding the drive chain over a plurality of freewheel sprockets which are spaced axially along the axis of the rear wheel hub as disclosed, for example, in U.S. Pat. Nos. 3,979,962 to Kebsch, 4,027,542 to Nagano and 4,038,878 to Dian. As is well known in the field and as is illustrated in FIG. 11 of the accompanying drawing, the typical parallelogram linkage mechanism of the prior art rear derailleur is formed by four links, that is, a stationary link (a) fixed to a mounting bracket (not shown) known as a rear end plate of a bicycle frame, a pair of spring-biased parallel links (b1) (b2) pivotally connected at their one ends to the stationary link (a), and a movable link (c) which is opposite to the stationary link (a) and pivotally connected to the other ends of the links (b1) (b2) to support a known tensioner (e) which, in turn, carries a guide roller (d) and a known tension roller (not shown). The parallelogram is resiliently transformable by operation of a known Bowden-type control cable (f), which consists of an outer tubular sheath (g) and an inner wire (j) passing through the former, within a certain predetermined range, for example, from the solid line position to the phantom line position or vice verse as shown in FIG. 11, so that the tensioner (e) carried by the movable link (c) can move with always maintaining parallel relation with respect to each of a plurality of sprockets of a multi-speed freewheel, to effect the desired chain shifting over the freewheel sprockets.
The conventional parallelogram type rear derailleur has the following disadvantages. The first is that the four link members (a) (b1) (b2) (c), cooperating coil springs (not shown in FIG. 11), and other component elements are all exposed to rain and dust and naturally get rusty and dusty, which causes an obstacle to a desired smooth transformation of the parallelogram and a short span of life.
Another disadvantage is that the parallelogram linkage mechanism hinders a simple and compact design of the rear derailleur and therefore a good appearance of the derailleur cannot be attained.
A further disadvantage is that not a single or mono type control cable but an expensive double type control cable (f) known as a Bowden cable must be employed to transform the parallelogram linkage. More particularly, the parallelogram can be transformed by varying a length of its diagonals. For that purpose, one end (h) of the tubular sheath or outer cable (g) should be fixed, at a position near one extremity of one of the diagonals, to one of the parallel links (link (b2) in the case of FIG. 11), while one end of the inner cable or wire (j) should be fixed, at a position near the other extremity of the diagonal, to a fixing member (k) provided on a pivot pin by means of which the movable link (c) and the other of the parallel links (link (b1) in the case of FIG. 11) are pivotally connected together, so that the length of the diagonal can be varied by an axial sliding motion of the inner wire (j) relative to the outer cable or sheath (g). A less expensive single type control cable cannot be utilized for this purpose. Additionally, it is apparent that the single type control cable requires less force in operation than the double type control cable, because a certain frictional resistance is present in the axial sliding motion of the inner wire (j) relative to the outer cable (g).
Accordingly, it is an object of the invention to eliminate the above-discussed disadvantages in the prior art bicycle derailleurs.
Another object of the invention is to provide an improved, novel structure applicable not only to a rear derailleur but also to a front derailleur.
A further object of the invention is to provide an improved bicycle derailleur structure which permits protection of important component elements from rain and dust for a long span of life without maintenance.
A still further object of the invention is to provide an improved bicycle derailleur structure which permits a very simple and good appearance.